Respiratory System
The
development of the respiratory system is continuous from the fourth week, when
the respiratory diverticulum appears, to term. The 24‐week potential viability
of a foetus (approximately 50% chance of survival) is partly because at this
stage the lungs have developed enough to oxygenate the blood. Limiters to oxygenation
include the surface area available to gaseous exchange, the vascularisation of
those tissues of gaseous exchange and the action of surfactant in reducing the
surface tension of fluids within the lungs.
Development
of the respiratory system includes not only the lungs, but also the conducting
pathways, including the trachea, bronchi and bronchioles. Lung development can
be described in five stages: embryonic, pseudoglandular, canalicular,
saccular and alveolar.
Although not
in use as gas exchange organs in utero, the lungs have a role in the
production of some amniotic fluid.
The
development of the respiratory system begins with the growth of an endodermal
bud from the ventral wall of the developing gut tube in the fourth week (Figure
32.1).
To separate
the lung bud from the gut tube two longitudinal folds form in the early tube of
the foregut, meet and fuse, creating the tracheoesophageal septum. This
division splits the dorsal foregut (oesophagus) from the ventral lung bud
(larynx, trachea hese structures remain in communication superiorly hrough the
laryngeal orifice.
Being
derived from the gut the epithelial lining is endodermal in origin, but as the
bud grows into the surrounding mesoderm reciprocal interactions between the
germ layers occur. The mesoderm develops into the cartilage and smooth muscle
of the respiratory conduction pathways.
In the fifth
week the tracheal bud splits and forms two lateral out-growths: the bronchial
buds. It is at this early stage we see the asymmetry of the lungs appear;
the right bud forms three bronchi and the left two. The bronchial buds branch
and extend, forming the respiratory tree of the three right lobes and two left
lobes of the lungs (Figure 32.1).
Up to week 5
the first period of lung development is known as the embryonic stage.
From 6 weeks
their development enters the pseudoglandular stage. The respiratory tree
continues to lengthen and divide with 16 20 generations of divisions by the end
of this stage (Figure 32.2). Histologically, the lungs resemble a gland at this
stage.
Epithelial
cells of the bronchial tree become ciliated and the beginnings of respiratory
elements appear. Cartilage and smooth muscle cells appear in the walls of the
bronchi. Lung‐specific type II alveolar cells (pneumocytes) begin to
appear. These are the cells that will produce surfactant.
The
pseudoglandular stage ends at approximately 16 weeks, by which time the entire
respiratory tree, including terminal bronchioles, has formed (Figure 32.2).
During the
next phase, known as the canalicular stage (17–24 weeks), the
respiratory parts of the lungs develop. Canaliculi (canals or tubes) branch out
from the terminal bronchioles. Each forms an acinus comprising the
terminal bronchiole, an alveolar duct and a terminal sac (Figure 32.2). This is
the primitive alveolus.
The duct
lumens become wider and the epithelial cells of some of the primitive alveoli
flatten to form type I alveolar cells (also known as type I pneumocytes,
or squamous alveolar cells). These will be the cells of gaseous exchange.
An invasion
of capillaries into the mesenchyme surrounding the primitive alveoli brings
blood vessels to the type I alveolar cells. Towards the end of the canalicular
stage some primitive alveoli are sufficiently developed and vascularised to
allow gaseous exchange, and a foetus born at this stage may survive with
intensive care support.
The saccular
stage (or terminal sac period, from 25 weeks to birth), describes the
continued development of the respiratory parts of the lungs. Type II
alveolar cells (also known as type II pneumocytes, great alveolar cells or
septal cells) begin to produce surfactant, a phospholipoprotein that
reduces the surface tension of the fluid in the lungs and will prevent collapse
of the alveoli upon expiration and improve lung compliance after birth.
During this
stage many more primitive alveolar sacs develop from the terminal bronchioles
and alveolar ducts. The blood air barrier between the epithelial type I
alveolar cells and endothelial cells of the capillaries develops in earnest,
and the surface area available to gaseous exchange begins to increase
considerably.
The final alveolar
stage (36 weeks onwards) begins a few weeks before birth and continues
postnatally through childhood. Alveoli increase in number and diameter enlarging
the surface area avail- able to gas exchange (Figure 32.2). The squamous (type
I alveolar) epithelial cells lining the primitive alveoli continue to thin
before birth, forming mature alveoli (Figure 32.3). Septation divides
the alveoli. Surfactant is produced in sufficient quantities for normal lung
function with birth. Continued development through child-hood will increase the
number of alveoli from 20–50 million at birth to around 400 million in the
adult lung (Table 32.1).
Two classes
of blood circulation are present in the lungs: pulmonary and bronchial.
Pulmonary arteries derive from the artery of the sixth pharyngeal arch and
accompany the bronchial tree as it branches, while the pulmonary veins lie more
peripherally. This part of the circulatory system is involved in gaseous
exchange, and until birth little blood flows through the pulmonary vessels. For
the changes to this circulatory system that occur at birth see Chapter 31.
Bronchial
vessels supply the tissues of the lung. These vessels are initially direct
branches from the paired dorsal aortae.
Clinical relevance
Respiratory distress syndrome (hyaline membrane
disease) caused by a lack of surfactant results in atelectasis (lung collapse).
This affects premature infants, and treatment options include a dose of
steroids given to the infant to stimulate surfactant production, or surfactant
therapy. Surfactant is administered to the infant directly down a tracheal
tube. These treatments together with oxygen therapy and the application of a
continuous positive airway pressure using a mechanical ventilator mean that the
prognosis is good in many cases.
Oesophageal atresia and tracheoeosphageal
fistulas are relatively common abnormalities. If the separation of the
trachea from the foregut is incomplete various types of communicating passages
may persist. This type of abnormality is often associated with other faults,
including cardiac defects, limb defects and anal atresia. It is also possible
that an oesophageal atresia will lead to polyhydramnios as the amniotic fluid
is not swallowed by the foetus, or pneumonia after birth as fluid may enter the
trachea through the fistula. Surgery is generally required.
Ectopic lung
lobes and abnormalities in the branching of the bronchial tree rarely produce
symptoms.
Congenital
cysts of the lung common infection sites and difficulty in breathing.
